Identification and characterization of the first pectin methylesterase gene discovered in the root lesion nematode Pratylenchus penetrans

Melting the wall: plant parasitism entails pectin modification

Phtheirospermum japonicum shows induced expression of PjPME and PjPMEI genes during haustoria development in rice and Arabidopsis with increased PME activity, which leads to the modulated cell wall during parasitism. Moreover, how PME and PMEI proteins interact and balance during haustoria development remains elusive.

Pectin modifications promote haustoria development in the parasitic plant Phtheirospermum japonicum

Parasitic plants are globally prevalent pathogens with important ecological functions but also potentially devastating agricultural consequences. Common to all parasites is the formation of the haustorium which requires parasite organ development and tissue invasion into the host. Both processes involve cell wall modifications. Here, we investigated a role for pectins during haustorium development in the facultative parasitic plant Phtheirospermum japonicum. Using transcriptomics data from infected Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), we identified genes for multiple P. japonicum pectin methylesterases (PMEs) and their inhibitors (PMEIs) whose expression was upregulated by haustoria formation. Changes in PME and PMEI expression were associated with tissue-specific modifications in pectin methylesterification. While de-methylesterified pectins were present in outer haustorial cells, highly methylesterified pectins were present in inner vascular tissues, including the xylem bridge that connects parasite to host. Specifically blocking xylem bridge formation in the haustoria inhibited several PME and PMEI genes from activating. Similarly, inhibiting PME activity using chemicals or by overexpressing PMEI genes delayed haustoria development. Our results suggest a dynamic and tissue-specific regulation of pectin contributes to haustoria initiation and to the establishment of xylem connections between parasite and host.

Genome-wide survey and evolutionary history of the pectin methylesterase (PME) gene family in the Dothideomycetes class of fungi

Once deposited in the plant cell wall, pectin undergoes demethylesterification by endogenous pectin methylesterases (PMEs), which play various roles in growth and development, including defense against pathogen attacks. Pathogen PMEs can alter pectin's methylesterification pattern, increasing its susceptibility to degradation by other fungal pectinases and thus playing a critical role as virulence factors during early infection stages. To investigate the evolutionary history of PMEs in the Dothideomycetes class of fungi, we obtained genomic data from 15 orders (79 species) and added genomic data from 61 isolates of Corynespora cassiicola. Our analyses involved maximum likelihood phylogenies, gene genealogies, and selection analyses. Additionally, we measured PME gene expression levels of C. cassiicola using soybean as a host through RT-qPCR assays. We recovered 145 putative effector PMEs and 57 putative non-effector PMEs from across the Dothideomycetes. The PME gene family exhibits a small size (up to 5 members per genome) and comprises three major clades. The evolutionary patterns of the PME1 and PME2 clades were largely shaped by duplications and recurring gene retention events, while biased gene loss characterized the small-sized PME3 clade. The presence of five members in the PME gene family of C. cassiicola suggests that the family may play a key role in the evolutionary success of C. cassiicola as a polyphagous plant pathogen. The haplogroups Cc_PME1.1 and Cc_PME1.2 exhibited an accelerated rate of evolution, whereas Cc_PME2.1, Cc_PME2.2, and Cc_PME2.3 seem to be under strong purifying selective constraints. All five PME genes were expressed during infection of soybean leaves, with the highest levels during from six to eight days post-inoculation. The highest relative expression level was measured for CC_29_g7533, a member of the Cc_PME2.3 clade, while the remaining four genes had relatively lower levels of expression.

Targeted transcriptomics reveals signatures of large-scale independent origins and concerted regulation of effector genes in Radopholus similis

The burrowing nematode, Radopholus similis, is an economically important plant-parasitic nematode that inflicts damage and yield loss to a wide range of crops. This migratory endoparasite is widely distributed in warmer regions and causes extensive destruction to the root systems of important food crops (e.g., citrus, banana). Despite the economic importance of this nematode, little is known about the repertoire of effectors owned by this species. Here we combined spatially and temporally resolved next-generation sequencing datasets of R. similis to select a list of candidates for the identification of effector genes for this species. We confirmed spatial expression of transcripts of 30 new candidate effectors within the esophageal glands of R. similis by in situ hybridization, revealing a large number of pioneer genes specific to this nematode. We identify a gland promoter motif specifically associated with the subventral glands (named Rs-SUG box), a putative hallmark of spatial and concerted regulation of these effectors. Nematode transcriptome analyses confirmed the expression of these effectors during the interaction with the host, with a large number of pioneer genes being especially abundant. Our data revealed that R. similis holds a diverse and emergent repertoire of effectors, which has been shaped by various evolutionary events, including neofunctionalization, horizontal gene transfer, and possibly by de novo gene birth. In addition, we also report the first GH62 gene so far discovered for any metazoan and putatively acquired by lateral gene transfer from a bacterial donor. Considering the economic damage caused by R. similis, this information provides valuable data to elucidate the mode of parasitism of this nematode.

The Root Lesion Nematode Effector Ppen10370 Is Essential for Parasitism of Pratylenchus penetrans

The root lesion nematode Pratylenchus penetrans is a migratory species that attacks a broad range of crops. Like other plant pathogens, P. penetrans deploys a battery of secreted protein effectors to manipulate plant hosts and induce disease. Although several candidate effectors of P. penetrans have been identified, detailed mechanisms of their functions and particularly their host targets remain largely unexplored. In this study, a repertoire of candidate genes encoding pioneer effectors of P. penetrans was amplified from mixed life stages of the nematode, and candidate effectors were cloned and subjected to transient expression in a heterologous host, Nicotiana benthamiana, using potato virus X-based gene vector. Among seven analyzed genes, the candidate effector designated as Ppen10370 triggered pleiotropic phenotypes substantially different from those produced by wild type infection. Transcriptome analysis of plants expressing Ppen10370 demonstrated that observed phenotypic changes were likely related to disruption of core biological processes in the plant due to effector-originated activities. Cross-species comparative analysis of Ppen10370 identified homolog gene sequences in five other Pratylenchus species, and their transcripts were found to be localized specifically in the nematode esophageal glands by in situ hybridization. RNA silencing of the Ppen10370 resulted in a significant reduction of nematode reproduction and development, demonstrating an important role of the esophageal gland effector for parasitism.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Host sunflower-induced silencing of parasitism-related genes confers resistance to invading Orobanche cumana

Orobanche cumana is a holoparasitic plant that attaches to host-plant roots and seriously reduces the yield of sunflower (Helianthus annuus L.). Effective control methods are lacking with only a few known sources of genetic resistance. In this study, a seed-soak agroinoculation (SSA) method was established, and recombinant tobacco rattle virus vectors were constructed to express RNA interference (RNAi) inducers to cause virus-induced gene silencing (VIGS) in sunflower. A host target gene HaTubulin was systemically silenced in both leaf and root tissues by the SSA-VIGS approach. Trans-species silencing of O. cumana genes were confirmed for 10 out of 11 target genes with silencing efficiency of 23.43%-92.67%. Knockdown of target OcQR1, OcCKX5, and OcWRI1 genes reduced the haustoria number, and silencing of OcEXPA6 caused further phenotypic abnormalities such as shorter tubercles and necrosis. Overexpression of OcEXPA6 caused retarded root growth in alfalfa (Medicago sativa). The results demonstrate that these genes play an important role in the processes of O. cumana parasitism. High-throughput small RNA (sRNA) sequencing and bioinformatics analyses unveiled the distinct features of target gene-derived siRNAs in O. cumana such as siRNA transitivity, strand polarity, hotspot region, and 21/22-nt siRNA predominance, the latter of which was confirmed by Northern blot experiments. The possible RNAi mechanism is also discussed by analyzing RNAi machinery genes in O. cumana. Taken together, we established an efficient host-induced gene silencing technology for both functional genetics studies and potential control of O. cumana. The ease and effectiveness of this strategy could potentially be useful for other species provided they are amenable to SSA.

New insights into the specificity and processivity of two novel pectinases from Verticillium dahliae

Pectin, the major non-cellulosic component of primary cell wall can be degraded by polygalacturonases (PGs) and pectin methylesterases (PMEs) during pathogen attack on plants. We characterized two novel enzymes, VdPG2 and VdPME1, from the fungal plant pathogen Verticillium dahliae. VdPME1 was most active on citrus methylesterified pectin (55-70%) at pH 6 and a temperature of 40 °C, while VdPG2 was most active on polygalacturonic acid at pH 5 and a temperature of 50 °C. Using LC-MS/MS oligoprofiling, and various pectins, the mode of action of VdPME1 and VdPG2 were determined. VdPME1 was shown to be processive, in accordance with the electrostatic potential of the enzyme. VdPG2 was identified as endo-PG releasing both methylesterified and non-methylesterified oligogalacturonides (OGs). Additionally, when flax roots were used as substrate, acetylated OGs were detected. The comparisons of OGs released from Verticillium-susceptible and partially resistant flax cultivars identified new possible elicitor of plant defence responses.

A new esophageal gland transcriptome reveals signatures of large scale de novo effector birth in the root lesion nematode Pratylenchus penetrans

BACKGROUND

The root lesion nematode Pratylenchus penetrans is a migratory plant-parasitic nematode responsible for economically important losses in a wide number of crops. Despite the importance of P. penetrans, the molecular mechanisms employed by this nematode to promote virulence remain largely unknown.

RESULTS

Here we generated a new and comprehensive esophageal glands-specific transcriptome library for P. penetrans. In-depth analysis of this transcriptome enabled a robust identification of a catalogue of 30 new candidate effector genes, which were experimentally validated in the esophageal glands by in situ hybridization. We further validated the expression of a multifaceted network of candidate effectors during the interaction with different plants. To advance our understanding of the "effectorome" of P. penetrans, we adopted a phylogenetic approach and compared the expanded effector repertoire of P. penetrans to the genome/transcriptome of other nematode species with similar or contrasting parasitism strategies. Our data allowed us to infer plausible evolutionary histories that shaped the effector repertoire of P. penetrans, as well as other close and distant plant-parasitic nematodes. Two remarkable trends were apparent: 1) large scale effector birth in the Pratylenchidae in general and P. penetrans in particular, and 2) large scale effector death in sedentary (endo) plant-parasitic nematodes.

CONCLUSIONS

Our study doubles the number of validated Pratylenchus penetrans effectors reported in the literature. The dramatic effector gene gain in P. penetrans could be related to the remarkable ability of this nematode to parasitize a large number of plants. Our data provide valuable insights into nematode parasitism and contribute towards basic understating of the adaptation of P. penetrans and other root lesion nematodes to specific host plants.

Biochemical Prospects of Various Microbial Pectinase and Pectin: An Approachable Concept in Pharmaceutical Bioprocessing

Both pectin and pectinase are vitally imperative biomolecules in the biotechnological sector. These molecules are a feasible non-toxic contrivance of nature with extensive applicative perception. Understanding pectic substances and their structure, unique depolymerization, and biochemical properties such as a catalytic mechanism and the strong interrelationship among these molecules could immensely enhance their applicability in industries. For instance, gaining knowledge with respect to the versatile molecular heterogeneity of the compounds could be considered as the center of concern to resolve the industrial issues from multiple aspects. In the present review, an effort has been made to orchestrate the fundamental information related to structure, depolymerization characteristics, and classification of pectin as well as the types and biochemical properties of pectinase. Furthermore, various production methods related to the optimization of the product and its significant contribution to the pharmaceutical industry (either pectinase or derived pectic substances) are described in this article.

An Expansin-Like Candidate Effector Protein from Pratylenchus penetrans Modulates Immune Responses in Nicotiana benthamiana

The root lesion nematode (RLN) Pratylenchus penetrans is a migratory species that attacks a broad range of crops. After the RLN is initially attracted to host roots by root exudates and compounds, it releases secretions that are critical for successful parasitism. Among those secretions are nematode virulence factors or effectors that facilitate the entry and migration of nematodes through the roots and modulate plant immune defenses. The recognition of the effectors by host resistance proteins leads to effector-triggered immunity and incompatible plant-nematode interactions. Although many candidate effectors of the RLN and other plant-parasitic nematodes have been identified, the detailed mechanisms of their functions and particularly, their host targets remain largely unexplored. In this study, we sequenced and annotated genes encoding expansin-like proteins, which are major candidate effectors of P. penetrans. One of the genes, Pp-EXPB1, which was the most highly expressed during nematode infection in different plant species, was further functionally characterized via transient expression in the model plant Nicotiana benthamiana and global transcriptome profiling of gene expression changes triggered by this candidate effector in plants. As a result of this investigation, the biological roles of Pp-EXPB1 in nematode parasitism were proposed, the putative cellular targets of the proteins were identified, and the molecular mechanisms of plant responses to the nematode-secreted proteins were outlined.